Diffusion Flames Based on a Laminar Spray Flame Library

The present paper investigates the structure of turbulent spray diffusion flames by means of numerical simulations. The flamelet model for turbulent diffusion flames has recently been extended to turbulent spray diffusion flames. The model is suitable for considering detailed chemical reactions through use of a laminar flame library consisting of structures of laminar gas diffusion flamelets that are characterized by the mixture fraction and its scalar dissipation rate. The focus of the present paper is the implementation of laminar spray diffusion flames for use in turbulent flame computations. Since the structure of laminar spray flames is considerably different from that of their gaseous counterparts, new criteria need to be developed for the implementation of these structures. The present paper presents characteristics of laminar spray flames and their consideration in turbulent flame computations. Both the model predictions (using either laminar gas flames or laminar spray flames) are compared to experimental results of a turbulent, axisymmetric free jet where an air-assisted methanol spray is injected into air. The new model is free of parameters that need to be adjusted to predict experimental data, and it correctly predicts the principal structure of the turbulent spray flame. Two reaction zones are identified where the turbulent model using laminar gas flames needs an artificial cut-off temperature to predict the low temperature regime where liquid is present. Thus, the use of laminar spray flames is required for the prediction of turbulent spray flame characteristics within the vaporization zone.